Turbid or translucent media present special problems for optical systems. For example, a medium such as biological tissue may transmit light efficiently, but may scatter the light substantially so that it is difficult to see or image through the tissue or to direct light through the tissue to a particular target. The difficulty is not primarily due to the absorption of light by the sample, but to the scattering that interferes with conventional image formation.
While the scattering may appear random, it is actually deterministic. This determinism can be used by advanced optical techniques so that it is possible to compensate for the scattering in the turbid medium. For example, Yaqoob et al. have used a holographic recording technique produce a phase conjugate mirror, in order to image through a turbid medium. See Yaqoob et al., “Optical Phase Conjugation for Turbidity Suppression in Biological Samples”, Nature Photonics 2, 110-115 (2008). In the system of Yaqoob et al., an image is projected through a turbid sample, which scatters the projected light into a lithium niobate crystal. A reference beam is also directed through the crystal, and the interference pattern or hologram generated by interference of the reference beam with the scattered light is recorded by the crystal. After a time, the reference beam is switched off, and a conjugate of the reference beam is passed through the crystal, so that a conjugate of the original scattered light is produced. The conjugate of the scattered light follows (in reverse direction) the traces of the original scattered light, passing through the turbid sample, back through the imaging optics, and to a camera where an image is recorded. The effect of the scattering is thus reversed, and it is possible to image through the turbid or translucent medium.
A disadvantage of prior systems is that the recording of the hologram may take several minutes. Especially in medical treatment applications, there is a need for faster generation of a conjugate beam, to compensate for possible motion of targets.